The present invention is directed to an optical ring resonator and the like having a wavelength tuning range which can be varied with accuracy.
Optical ring resonators are of great interest in the telecommunication industry because of their ability to provide cross-connect architectures and because they can be made very compact in size. Other technologies that have been used to provide cross-connect architectures include thin-film interference filters, fiber gratings and arrayed waveguide gratings.
Cross-connect waveguide architecture is described in International Patent Appln. No. WO 00/50938, entitled xe2x80x9cVertically Coupled Optical Resonator Devices Over a Cross-Grid Waveguide Architecturexe2x80x9d.
One example of a cross-connect architecture using optical ring resonators is shown in FIG. 1, and is discussed hereafter.
An optical semiconductor resonator 7 has plurality of microcavity resonators 5 and input and output waveguides 1, 3 formed from semiconductor materials. The input 1 and output 3 waveguides are arranged so that a portion of each of the two waveguides is disposed adjacent to the microcavity resonator 5. Light propagating in the input waveguide 1 with a wavelength on resonance with the resonance wavelength of the microcavity resonator 5 is coupled to the microcavity resonator 5, and from the microcavity resonator 5 the light is coupled to the output waveguide, by way of example, n1, for transmission therefrom. Light propagating in the input waveguide 1 with a wavelength that is off resonance with the microcavity resonator 5 is not coupled to the microcavity resonator 5, but continues to propagate in the input waveguide 1 for output therefrom. Consequently, a resonator can serve as a wavelength-specific routing device which guides particular wavelengths of light from an input path to one of several output paths.
It will be appreciated that the terms xe2x80x9cinputxe2x80x9d and xe2x80x9coutputxe2x80x9d are used for convenience, and that light could be transmitted in the opposite manner, that is, from the xe2x80x9coutputxe2x80x9d waveguide to the xe2x80x9cinputxe2x80x9d waveguide.
The resonance wavelength for a ring resonator is a direct function of the ring resonator""s structure, and can be given as:
xcexi=2xcfx80Rn/ixe2x80x83xe2x80x83(1)
where xcexi is the resonance wavelength, R is the resonator radius (for a circular resonator), n is the resonator""s effective index of refraction, and i is any positive integer.
If the resonator is not circular, the resonant wavelength is given by the equation (2):                     λ        =                              L            ⁢                          xe2x80x83                        ⁢            n                    m                                    (        2        )            
In equation 2 L is the resonator""s length, n is the effective index of refraction of the optical signal, and m is an integer of value 1 or greater.
Resonator operation can be enhanced if the resonator""s operating wavelength can be varied, as that allows modification of the resonator""s switching behavior. For example, a user can select which wavelengths of light transmitted by a first waveguide are coupled to the resonator and to a second waveguide by changing the resonator""s resonance wavelength to match the wavelength of light sought to be routed.
There are several ways to alter a resonator""s index of refraction and so control the resonator""s operating wavelength. In accordance with equations (1) and (2), a resonator""s resonance wavelength is related to the resonator material and its index of refraction, so changing the resonator index of refraction leads to a corresponding change in the resonator resonance wavelength. Alternatively, the resonator""s size (i.e., radius) will determine the resonance wavelength.
Certain materials used in ring resonators have indices of refraction which vary with temperature. A ring resonator made from such a material could be thermally tuned. Changing the ring resonator""s temperature will alter the resonator""s index of refraction and size, as discussed in greater detail below, and thus produce a corresponding change in the resonance wavelength.
Another way to control a ring resonator""s resonance wavelength is to inject current into the resonator ring. Some of the semiconductor materials that can be used in ring resonators exhibit electro-optic behavior. A material having electro-optic properties experiences a change in its index of refraction when an electric field is applied thereto. A ring resonator constructed from an electro-optic semiconductor material can therefore be tuned through the application of a suitable electric field.
As already noted, ring resonators are frequently employed as part of the cross-connect architecture of optical networks. Ring resonators are well-suited for use as telecommunications systems switching devices in Wave Division Multiplexing (xe2x80x9cWDMxe2x80x9d) systems, various types of which systems will be discussed later on. These systems efficiently transmit data by simultaneously sending several different wavelengths of light over a single optical fiber or waveguide and then, at the appropriate point, separating (de-multiplexing) the combined signals into individual optical fibers or waveguides and routing those signals to their desired end-points or destinations.
FIG. 1 depicts a typical network architecture based on ring resonators, namely, an Mxc3x97N optical data network cross-connect 7. In cross-connect 7 each resonator 5 can take the signal coming from a horizontal input waveguide 1, and couple it into a vertical output waveguide 3, provided the wavelength of the optical signal in the input waveguide 1 is on-resonance with the resonance wavelength of the resonator 5. On the other hand, if the wavelength of the optical signal in the input waveguide 1 and the resonance wavelength of the resonator 5 are different, the optical signal remains undisturbed, i.e., does not couple to the resonator 5 in the input waveguide 1 and thus can travel toward and encounter a second resonator 5 downstream along the optical path of that waveguide 1.
Each of the M input waveguides 1 can be a long-distance transmission medium (i.e., fiber-optic cable or waveguide) which simultaneously carries a number of different wavelength signals between widely-separated points. The N output waveguides 3 may connect to optic fibers which carry a particular wavelength(s) of light between the long-distance transmission medium and a single device or user. Incidentally, it should be understood that while the foregoing discussion refers to optical fibers, the, input and/or output lines 1, 3, also could be any other suitable optical transmission devices, including by way of non-limiting example, waveguides.
Since the different wavelengths of light which are carried by each of the M input waveguides 1 are intended for different destinations, it is necessary to separate and suitably route each of those different wavelengths of light. As noted above, ring resonators 3 perform this routing function quickly and efficientlyxe2x80x94since each ring resonator 5 can couple a particular, wavelength of light traveling in an input waveguide 1 to an output wave guide 3, ring resonators 5 can be used to xe2x80x9cpick offxe2x80x9d the different wavelengths of light from a multi-wavelength optical signal, e.g., a WDM signal.
One common type of cross-connect is a multiplexer (MUX)/demultiplexer (deMUX). A MUX/deMUX is a cross-connect that links a multi-wavelength optical waveguide carrying N wavelengths of light to a total of N waveguides. Thus, in a MUX/deMUX, a single multi-wavelength waveguide will have a total of N corresponding ring resonators.
If the ring resonators used in a cross-connect can only separate out a, single wavelength of light, it will be necessary to provide the cross-connect with Mxc3x97N resonators. However, if the ring resonators can be tuned sufficiently, each of the ring resonators could separate out multiple wavelengths of light, and so some resonators could be omitted and the cross-connect structure could be simplified.
To be useful to the telecommunication market, resonators should meet two basic requirements, namely, they should be small in size and they should have a high tunability range.
Small size is desirable for two reasons. First, small resonators require less wafer real estate, which reduces costs. Second, small resonators have large free spectral range (FSR) characteristics. A resonator""s FSR is given as:                     FSR        =                  λ          ⁢                      xe2x80x83                    ⁢                      λ                          (                              2                ⁢                                  xe2x80x83                                ⁢                π                ⁢                                  xe2x80x83                                ⁢                Rn                            )                                                          (        3        )            
Here, xcex is the wavelength of the optical signal, R is the radius of the resonator and n is the effective refractive index material through which the optical signal propagates.
A large FSR may be preferred because it allows for a higher number of optical channels to be multiplexed in a single fiber, which better uses the fiber""s optical bandwidth. FIG. 3A depicts the spectral response and optical channel accommodation for a 10 xcexcm FSR resonator having architecture that allows for approximately six GHz channels in the 1.55 xcexcm telecommunication window. In contrast, a 40 xcexcm FSR resonator architecture could accommodate approximately twenty-five different optical channels, as illustrated in FIG. 3B. A compact resonator (or equivalently a resonator with a small radius) can be constructed in part by using strongly-confined waveguides (the term xe2x80x9cstrongly-confinedxe2x80x9d refers to a waveguide having a substantial difference in the index of refraction between core and cladding regions). Strongly-confined waveguides are useful because they are able to guide light around sharp bends.
There are a number of reasons why high resonator tunability is desirable.
In today""s technology, resonators with the dimensional precision required to insure that the resonators perform as required cannot easily be manufactured. Resonator size is important because as shown above in connection with Equations 1 and 2, a resonator""s radius directly affects both the resonator""s resonance wavelength and the resonator""s FSR. The resonator""s resonance wavelength is particularly important because it must comply with the ITU grid, which is a telecommunication standard that divides telecommunications windows into optical channels that are typically separated by fractions of a nanometer. If the resonator is to be used in a telecommunications system, the resonator""s wavelength must strictly conform to the ITU grid standard.
Using existing microfabrication technology, it is difficult to control the resonator radius with the accuracy necessary to assure that its resonance wavelength is precisely tuned to a wavelength lying on the ITU grid. This problem arises because the resonance wavelengths of an optical resonator are inversely related to the resonator""s size. This means that the resonance wavelength of a small resonator will be much more sensitive to variations in resonator radius than that of a large resonator.
By way of example, a deviation of just 10 nm in the radius of a nominally 10 xcexcm radius resonator (and this 0.1% deviation-presses the limits of what can be achieved using optical lithography) results in a resonator wavelength resonance deviation of 1.55 nm from the nominal wavelength for which the resonator was designed. A deviation of that magnitude is undesirable and in fact may be unacceptable in today""s telecommunication networks, since those networks have channel spacings well below 1 nm. Some form of wavelength tuning is therefore essential to reposition the resonance wavelength onto the ITU grid.
Such manufacturing deviations might, however, be tolerable if the resonator could be tuned sufficiently to change the resonator""s wavelength resonance to compensate for those variations. Known tuning techniques, which will be discussed in detail below, do not provide a sufficient tuning range to compensate for such manufacturing variations.
Increased resonator tunability is also desirable because it enables a network administrator to reconfigure dynamically the network during operation, without interrupting service, according to usage considerations and the demands of their clients. FIGS. 2A and 2B show an example of a network reconfigured from the arrangement shown in FIG. 2A to that shown in FIG. 2B. By selectively tuning each resonator 5 to a desired wavelength, various wavelengths present in the input waveguide 1 may be routed to the various output waveguides 3. That selective reconfiguration may change the paths of the signals with wavelengths xcex1, xcex2 and xcex3.
There are several techniques by which a resonator may be tuned. The optical resonance wavelength is a function of both the resonator geometry and the waveguide refractive index, as set forth previously in Equation. 1. Therefore, to change the resonance wavelength, either the index of refraction or the physical optical path length (for example, given in Equation 1 as n and R, respectively) must be altered.
The index of refraction of the waveguide material can be altered by changing the waveguide""s temperature (thermal tuning), injecting current (current tuning) into the waveguide, or applying voltage to the waveguide (electro-optic tuning).
For thermal tuning, the resonance wavelength shift can be expressed as:                               Δ          ⁢                      xe2x80x83                    ⁢          λ                =                  λ          ⁢                      xe2x80x83                    ⁢                                    Δ              ⁢                              xe2x80x83                            ⁢              n              ⁢                              xe2x80x83                            ⁢              Δ              ⁢                              xe2x80x83                            ⁢              R                        nR                                              (        4        )            
Here xcex is the wavelength of the optical signal, An is the change in the resonator material""s index of refraction, xcex94R is the change in the resonator""s radius, n is the effective index of refraction of the resonator material, and R is the resonator""s radius.
Thermal tuning is discussed in Rafizadeh, D., et al., xe2x80x9cTemperature Tuning of Microcavity Ring and Disk Resonators at 1.5-xcexcmxe2x80x9d, IEEE publication number 0-7803-3895-2/19 (1997). Rafizadeh discloses that the thermal tuning coefficient of a GaAs-based 10.5 xcexcm-diameter disk resonator has been experimentally observed to be 1.3 nm/10xc2x0 C.
In the case of either current injection or electro-optical tuning, the resulting change in resonance wavelength is:                               Δ          ⁢                      xe2x80x83                    ⁢          λ                =                  λ          ⁡                      (                                          Δ                ⁢                                  xe2x80x83                                ⁢                n                            n                        )                                              (        5        )            
Again, xcex is the wavelength of the optical signal, xcex94n is the change in the resonator material""s index of refraction, and n is the effective index of refraction of the resonator material.
A common semiconductor waveguide construction for implementing either current injection or electro-optic tuning involves doping the upper cladding with p-type dopant, the waveguide core with low or intrinsic dopant, and the lower cladding and substrate with n-type dopant. If electric contact is made to the upper (p-type) and lower (n-type) waveguide layers, the resulting p-i-n junction may then be operated in forward- or reverse-bias mode. Under forward bias, a change in the index of refraction of the waveguide core may be induced through current injection. Under reverse bias, a high electrical field can be formed across the intrinsic waveguide core and a refractive index change can result through the electro-optic effect. Both of these effects provide only a relatively small tuning effect.
Although tuning by changing the index of refraction using either current injection or the electro-optic effect can provide very high switching speeds (in the microsecond and nanosecond regimes, respectively), these techniques can only tune a resonator over a very limited range of wavelengths, on the order of several nanometers. This is not sufficient, however, since tuning over a spectral range of at least 10-20 nm is desirable for many telecommunication applications.
Thermal tuning offers the possibility of a much greater tuning range than current injection and the electro-optic effect, although at somewhat slower speeds (expected to be in the sub-millisecond or even millisecond range).
While general thermal tuning of telecommunications devices, such as laser diodes or arrayed waveguide gratings, is known, such thermal tuning is global, not local. That is, since many such telecommunications devices are highly sensitive to temperature changes, global temperature control of the entire device may be provided to improve wavelength stability, as well as to effect tuning. For such components, global temperature control is achieved by mounting the substrate to a temperature controlling device, such as a thermoelectric cooler (TEC).
A specific form of temperature control has been used in certain thermo-optic switches which incorporate Mach-Zehnder interferometers (MZI). As described in Lai, Q., et al. xe2x80x9cLow-Power Compact 2xc3x972 Thermooptic Silica-on-Silicon Waveguide Switch With Fast Responsexe2x80x9d, IEEE publication number 1041-1135/98 (1998), an electric heater is fabricated over one optical waveguide of the MZI. Switching is achieved by heating the arm of the MZI to cause a temperature change of 40 xc2x0 C. and-alter the arm""s index of refraction, thereby inducing a xcfx80 phase shift into the waveguide arm. These MZI devices are 5 mm long and so are far too large for practical use in an optical system.
Consequently, while ring resonators can be tuned to vary the wavelength of light coupled between input and output waveguides, there is additional strong demand for resonators which can be tuned across a wider range of wavelengths.
More particularly, there is a need for a ring resonator that can be tuned such that the resonator""s operating wavelength varies by at least approximately an order of magnitude more than the 1-2 nm tuning range currently achievable.
The present invention is directed in part toward localized thermal tuning of optical resonators. This is in marked contrast to the conventional thermal tuning of telecommunications devices, already discussed, which use global thermal tuning.
Optical resonators with compact dimension (on the order of several tens-of microns or less) offer the promise of high integration densities. As discussed previously, some form of tuning is critically important for these devices, due to their sensitivity to fabrication tolerances as well as to enable network reconfiguration. Thermal tuning is a very important physical effect because it can produce relatively large changes in resonance wavelength compared to other tuning methods. This invention achieves thermal tuning of optical resonators by delivering localized thermal energy to the resonator cavity. This allows single resonators to be tuned individually, thereby enabling high device integration densities to be realized.
An important feature of this invention is the use of a compact electric heater which can provide efficient, localized heating of the optical resonator. This heater could take the form of a forward-biased or p-i-n junction, or a surface resistance heater.
Among the benefits of this invention is the avoidance of absorption loss due to free carriers or change in the strength of the optical field confinement within the waveguide guiding region. This is possible because heating is accomplished in a manner which does not interfere with resonator operation.
Another benefit to this invention is that wavelength tuning and switching can be effected over a wide wavelength spectrum, especially if GaAs- or InP-based waveguides are used.
Still another advantage to this invention is that single mode GaAs-base P-I-N optical waveguide materials can be designed with thicker guiding region compared to InP-based waveguides. This enhances coupling efficiency.
By virtue of their small size, devices constructed in accordance with this invention are highly integratable in a large matrix on a single small substrate chip.
The present invention provides a thermally tunable resonator using a self-feeding resonator body with a resonance wavelength, a trench formed inside the resonator body, and a disk formed inside the trench, the disk having semiconductor layers which are doped to form a forward biased junction. A first electrode is positioned atop the disk and in electrical contact with the forward biased junction, this electrode not covering the resonator body, and a second electrode is formed beneath and in electrical contact with the forward biased junction. When current is applied through the electrodes to the disk, heat is generated, and this changes the resonance wavelength of the resonator body.
Another aspect of this invention relates to a thermally tunable resonator having a self-feeding resonator body with a resonance wavelength and a heater. The heater has first and second contact pads that connect to a resistance disposed above the resonator body. When current is applied through the contact pads to the resistance, heat is generated, changing the resonator body""s resonance wavelength.
This invention also involves a thermally tunable resonator which includes a self-feeding resonator body with a resonance wavelength, a heater having first and second contact pads, and a resistance connected to the contact pads and disposed above the resonator body. A temperature sensor senses the temperature of the resonator body, and when current is applied through the contact pads to the resistance, heat is generated, changing the resonance wavelength of the resonator body.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indicated in the claims.